This invention relates to devices, implants and prostheses used in orthopedic surgery, and, more particularly, to bone plates used in Triple Pelvic Osteotomy (TPO), to join reformed bone and thus promote healing.
Bone plates have been used to repair fractured or opened bones at least since the time of the Incas. The innovations in this field have involved plate materials, plate form, and the means of fixing the plate across the bone fragments to be joined.
In an effort to deal with the fixation of a bone plate, a compressive screw system, also known as the DCS bone plate system, was developed and in use in trauma surgery for many years. The procedures for use of this system are well documented by the AO Institute, an institute having as one of its goals, the promotion of new orthopedic surgical procedures. This system included a bone plate having slots communicating there through. A land in which the slot is wider at one end defines a stepped surface adjacent the portion of the slot that extends through the bone plate. The stepped surface is generally cut with a spherical endmill, thus creating a spherical stepped surface.
In a still further development, bone plates have been developed having individual threaded apertures and non-threaded apertures interspersed along the length of the plate. In this and other designs, the distance between holes has become a standard. Although an improvement over the inserts noted above, the locking positions are pre-defined, and only available in limited locations, which also reduce surgical flexibility.
In yet another solution, PCT application no. WO01/54601 combines the features of the DCS system discussed above with a locking screw. This design combines the features of the DCS system with a locking screw. Such a system is known as the combi-slot. In this design, the stepped surface of the slot is generally ramped or tapered so as to be deeper at one end than at another. This enables the positioning and selective fixing of the bone plate for compressing two bone fragments together with a preload created by wedging action. In this manner, the bones are placed in a position that the surgeon believes would best promote healing.
While patent application no. WO01/54601 has proven advantageous because screws can be locked to the plate, the presence of an unthreaded slot limits the users ability to have multiple orientations for the screw.
In a further development, the AO Institute has studied and proposed the use of endpegs which are rigidly fixed in the extreme ends of the bone plate. Such an arrangement has been shown to better resist the flexing of the bone than use of a bone screw alone. Flexing can otherwise loosen the connection between the bone plate and bone in other bone plate systems.
In another development, German patent DE 4341980 A1, published on Jun. 14, 1995, describes a bone plate 2 having an elongated slot 8 in which the sidewalls of the long sides of the slot are not parallel and are further provided with an internal thread 9. Corresponding bone screws 3 or inserts 6 have a head 5 with an external taper 4 and thus can be fixed into any point along the length, but to various depths of penetration. Therefore, the final configuration upon fixing is indeterminate and, due to the small amount of contact between the threads of the insert or screw and the slot, as well as the fact that the screw will be able to slide in one direction, the design does not appear to lend itself to reliable fixing.
U.S. Pat. No. 5,324,290 shows a complex hone plate having slots with countersunk circular recessed cut at intervals along the slot (a similar arrangement is shown in U.S. Pat. No. 4,696,290). It further shows the bone plate torqued against the bone so as to at least marginally conform to the shape of the bone (see FIG. 2). Other patents of interest include U.S. Pat. Nos. 3,716,050, 3,659,595, 5,681,311, 5,261,910, and 5,364,399, all showing combinations of conventional slots and recesses which do not fully accommodate a bone screw having a threaded head.
In a Triple Pelvic Osteotomy, it is necessary to treat a subluxed hip in a canine, which is a genetic abnormality. This is when the femoral head is not sufficiently covered (less than 50% coverage) by the rim of the acetabulum (see pg. 472 of Appendix A attached and incorporated herein by reference thereto).
Consequently, a TPO plate is made up essentially of two plates that are non-parallel to each other, being rotated with respect to each other about an axis, and fixed together by an offset web. Different cases necessitate different angular changes to best cover the femoral head. If a femoral head is covered 30 degrees instead of 50, for example, one would need to use a 20 degree TPO plate to reach 50% coverage, and so on. The plate positions the bone at the correct anatomical angle.
In another product variation, expandable, lockable inserts enter into the slots of a standard bone plate. When the bone screw passes through one of these inserts and is torqued down, the insert expands and locks the screw in place. However, this insert is locked in a secondary operation. This is not desirable because this requires more operating room time and adds complexity to the procedure. Further, the inserts must be added in the specific location before the plate is fixed to the bone and cannot be subsequently inserted. This limits the choice of placement during surgery if the need arises.
Also, the above insert design relies on a friction lock via contact between two surfaces. Friction locks are not reliable and come lose more easily than threaded locked holes. The result of such a design is inferior to that of the threaded plate and screw designs discussed below.
In prior art TPO plates, it is known that the bone screws can come lose, causing pain and/or requiring corrective surgery. What is needed, therefore, is a TPO plate which can be firmly fixed to the bone in a manner to minimize the likelihood of loosening of the bone screws. In addition, what is needed is a TPO plate where the holes are located to achieve the best anatomical location of the screws in the bone.
What is needed is a bone plate that provides the surgeon with multiple orientations for the locking screw and thus, plate placement, while reliably and permanently fixing the bone plate to the bone fragments in any hole position. More specifically, what is needed is a bone plate that provides this choice of plate placement while reliably and permanently fixing the bone plate to the bone fragments, in any hole position.
What is needed is a bone plate with holes that create at least unidirectional compression.